Search

Adaptation of cell populations to environmental changes is mediated by phenotypic variability at the single-cell level. Enzyme activity is a key factor in cell phenotype and the expression of the alkaline phosphatase activity (APA) is a fundamental phytoplankton strategy for maintaining growth under phosphate-limited conditions. Our aim was to compare the APA among cells and species revived from sediments of the Bay of Brest (Brittany, France), corresponding to a pre-eutrophication period (1940’s) and a beginning of a post-eutrophication period (1990’s) during which phosphate concentrations have undergone substantial variations. Both toxic marine dinoflagellate Alexandrium minutum and the non-toxic dinoflagellate Scrippsiella acuminata were revived from ancient sediments. Using microfluidics, we measured the kinetics of APA at the single-cell level. Our results indicate that all S. acuminata strains had significantly higher APA than A. minutum strains. For both species, the APA in the 1990’s decade was significantly lower than in the 1940’s. For the first time, our results reveal both inter and intraspecific variabilities of dinoflagellate APA and suggest that, at a half-century timescale, two different species of dinoflagellate may have undergone similar adaptative evolution to face environmental changes and acquire ecological advantages.

International audience; Biofilms of heterotrophic bacteria cover organic matter aggregates and constitute hotspots of mineralization, primarily acting through extracellular hydrolytic enzyme production. Nevertheless, regulation of both biofilm and hydrolytic enzyme synthesis remains poorly investigated, especially in estuarine ecosystems. In this study, various bioassays, mass spectrometry and genomics approaches were combined to test the possible involvement of quorum sensing (QS) in these mechanisms. QS is a bacterial cell–cell communication system that relies notably on the emission of N-acylhomoserine lactones (AHLs). In our estuarine bacterial collection, we found that 28 strains (9%), mainly Vibrio, Pseudomonas and Acinetobacter isolates, produced at least 14 different types of AHLs encoded by various luxI genes. We then inhibited the AHL QS circuits of those 28 strains using a broad-spectrum lactonase preparation and tested whether biofilm production as well as β-glucosidase and leucine-aminopeptidase activities were impacted. Interestingly, we recorded contrasted responses, as biofilm production, dissolved and cell-bound β-glucosidase and leucine-aminopeptidase activities significantly increased in 4%–68% of strains but decreased in 0%–21% of strains. These findings highlight the key role of AHL-based QS in estuarine bacterial physiology and ultimately on biogeochemical cycles. They also point out the complexity of QS regulations within natural microbial assemblages.

Phosphorus (P) limitation of phytoplankton growth is known to affect the accumulation and release of carbohydrates (CHO) by micro-algae. However, relatively little is known about the fate of algal exudates, notably their bacterial degradation. The CHO chemical characterization is also not exhaustive, especially in ‘functional’ pools relevant for phytoplankton physiology (particulate reserve [R] or structural [S] CHO) and for bacterial degradation (dissolved mono- [MDCHO] and polysaccharides [P-DCHO]). In this study, we investigated how P depletion and repletion affect the CHO composition in diatomThalassiosira weissflogiicultures, and the shortterm response of free and diatom-attached bacteria in terms of abundance and potential βglucosidase activity (βGlc). The bacterial inoculum was composed of the bacterial consortiums of diatom precultures and a natural bacterial community from the Bay of Brest. P depletion favored CHO accumulation in diatom cells, mainly as R i.e. soluble CHO accumulated in cytoplasm, but also as S, polysaccharides linked to the cell wall. The R:S ratio was high in the present diatom cultures. The high M-DCHO observed in P-deplete cultures (twice that of P-replete cultures) when P-DCHO remained quite similar is explained both by active polysaccharide hydrolysis (very high potential βGlc of attached bacteria) and reduced uptake of M-DCHO by Pdepleted bacteria. P depletion of heterotrophic bacteria favors labile CHO accumulation, which may affect particle potential aggregation. However, the remarkably constant M-DCHO concentration over time for both conditions suggests tight coupling between phytoplankton accumulation, release, polymer hydrolysis and monomer uptake by bacteria.

The occurrence of alkaline phosphatase activity (APA) that hydrolyses organic phosphorus into phosphate (PO4) is commonly related to PO4 deficiency of oceanic, coastal and fresh waters. APA is almost never investigated in PO4-rich estuaries, since very low activities are expected to occur. As a consequence, microbial mineralization of organic phosphorus into PO4 has often been ignored in estuaries. In this study, we examined the importance of potential APA and the associated microbial dynamics in two estuaries, the Aulne and the Elorn (Northwestern France), presenting two different levels of PO4 concentrations. Unexpected high potential APA was observed in both estuaries. Values ranged from 50 to 506 nmol L−1 h−1, which range is usually found in very phosphorus-limited environments. High potential APA values were observed in the oligohaline zone (salinity 5–15) in spring and summer, corresponding to a PO4 peak and a maximum bacterial production of particle-attached bacteria. In all cases, high potential APA was associated with high suspended particulate matter and total particulate phosphorus. The low contribution of the 0.2–1 μm fraction to total APA, the strong correlation between particulate APA and bacterial biomass, and the close relationship between the production of particle-attached bacteria and APA, suggested that high potential APA is mainly due to particle-attached bacteria. These results suggest that the microbial mineralization of organic phosphorus may contribute to an estuarine PO4 production in spring and summer besides physicochemical processes.

Toxic algae are a worldwide problem threatening aquaculture public health and tourism Alexandrium a toxic dinoflagellate proliferates in Northwest France estuaries (i e the Penze estuary) causing Paralytic Shellfish Poisoning events Vegetative growth and in particular the role of nutrient uptake and growth rate are crucial parameters to understand toxic blooms With the goal of modelling in situ Alexandrium blooms related to environmental parameters we first try to calibrate a zero-dimensional box model of Alexandrium growth This work focuses on phosphorus nutrition Our objective is to calibrate Alexandrium minutum as well as Heterocapsa triquetra (a non-toxic dinoflagellate) growth under different rates of phosphorus supply other factors being optimal and constant Laboratory experiments are used to calibrate two growth models and three uptake models for each species Models are then used to simulate monospecific batch and semi continuous experiments as well as competition between the two algae (mixed cultures) Results show that the Droop growth model together with linear uptake versus quota can represent most of our observations although a power law uptake function can more accurately simulate our phosphorus uptake data We note that such models have limitations in non steady-state situations and cell quotas can depend on a variety of factors so care must be taken in extrapolating these results beyond the specific conditions studied (C) 2010 Elsevier BV All rights reserved

Alexandrium catenella (Whedon et Kof.) Balech has exhibited seasonal recurrent blooms in the Thau lagoon (South of France) since first reported in 1995. Its appearance followed a strong decrease (90%) in phosphate (PO(4)3-) concentrations in this environment over the 1970-1995 period. To determine if this dinoflagellate species has a competitive advantage in PO(4)3--limited conditions in terms of nutrient acquisition, semicontinuous cultures were carried out to characterize phosphorus (P) uptake by A. catenella cells along a P-limitation gradient using different dilution rates (DRs). Use of both inorganic and organic P was investigated from measurements of 33PO(4)3- uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (Q(P) and Q(C)). Shifts in trends of Q(P)/Q(C) and Q(P) per cell (Q(P center dot cell-1)) along the DR gradient allowed the definition of successive P-stress thresholds for A. catenella cells. The maximal uptake rate of 33PO(4)3- increased strongly with the decrease in DR and the decrease in Q(P)/Q(C), displaying physiological acclimations to PO(4)3- limitation. Concerning maximal APA per cell, the observation of an all-or-nothing pattern along the dilution gradient suggests that synthesis of AP was induced and maximized at the cellular scale as soon as PO(4)3- limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO(4)3- threshold between 0.4 and 1 mu M. As lower PO(4)3- concentrations are regularly observed during A. catenella blooms in Thau lagoon, a significant portion of P uptake by A. catenella cells in the field may come from organic compounds.

Seasonal variations in nutrient inputs are described for the main rivers (Loire and Vilaine) flowing into the northern Bay of Biscay. The river plumes are high in N/P ratio in late winter and spring, but not in the inner plume during the summer. Conservative behavior results in most nutrients entering the estuary and eventually reaching the coastal zone. Temporal and spatial aspects of phytoplankton growth and nutrient uptake in the northern Bay of Biscay distinguish the central area of salinity 34 from the plume area. The first diatom bloom appears offshore in late winter, at the edge of the river plumes, taking advantage of haline stratification and anticyclonic "weather windows." In spring, when the central area of the northern shelf is phosphorus-limited, small cells predominate in the phytoplankton community and compete with bacteria for both mineral and organic phosphorus. At that period, river plumes are less extensive than in winter, but local nutrient enrichment at the river mouth allows diatom growth. In summer, phytoplankton become nitrogen-limited in the river plumes; the central area of the shelf is occupied by small forms of phytoplankton, which are located on the thermocline and use predominantly regenerated nutrients.

The understanding of the dominance of one species with respect to others is a pertinent challenge in HAB growth dynamics studies and the nutrient supply mode is one of the factors potentially involved. The competition for phosphorus (P) between a toxic species, Alexandrium minutum, and a non-toxic species, Heterocapsa triquetra, was studied (1) along a gradient of P depletion, (2) testing different P depletion degrees before a single PO4 supply and (3) experimenting different PO4 supply frequencies. In conditions Of PO4 depletion, H. triquetra stopped growing after two days both in monospecific and mixed batch cultures whereas A. minutum grew progressively from day 2 until the end of the experiment. This time-lag growth of A. minutum is associated to its ability to store P intracellularly and then mobilize it for cell division when P depletion becomes severe. Heterocapsa triquetra outcompeted A. minutum when it was submitted to less than three days of P depletion before the pulse. In contrast, A. minutum outcompeted H. triquetra after more than three days of depletion. This transition was related to the capacity for A. minutum to increase its cell PO4 uptake rate in a higher proportion to face potential PO4 supply. As a result of this physiological acclimatation to P starvation, A. minutum consumed the whole PO4 pulse supplied after 3 to 10 days of P depletion. This resulted in a reduction of H. triquetra growth. These two acclimatations were confirmed in a P limited semi-continuous culture experiment testing several PO4 supply frequencies (1, 2, 4, 6 day intervals). These experiments revealed that A. minutum is a "storage specialist" species for P, which uptakes PO4 pulses for luxury consumption, survives depletion periods and, then, utilizes P for cell growth. In contrast, H. triquetra is more a "velocity adapted" species, which utilizes PO4 just after supply to increase their cell division rate. (C) 2008 Elsevier B.V. All rights reserved.

International audience; Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)-limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (Ks50.26 0.03 and 0.31 0.04 lmol .L 1, respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (rNO3=rNH4 50.42-0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N-sufficient cells (Imax~0.9 and Ki50.31-0.56 lmol .L 1). In the case of N assimilation, ammonium was also the preferred form in N-deficient cells (rassim NO3=rassim NH4 50.54-0.72), whereas in N-sufficient cells, both N sources were equally preferred (rassim NO3=rassim NH4 50.90-1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest-growing N-deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.

Previous studies conducted on the continental shelf in the Southeast Bay of Biscay influenced by Gironde waters (one of the two largest rivers on the French Atlantic coast) showed the occurrence of late winter phytoplankton blooms and phosphorus limitation of algal growth thereafter. In this context, the importance of dissolved organic phosphorus (DOP) for both algae and bacteria was investigated in 1998 and 1999 in terms of stocks and fluxes. Within the mixed layer, although phosphate decreased until exhaustion from winter to spring, DOP remained high and phosphate monoesters made up between I I to 65% of this pool. Total alkaline phosphatase activity (APA, V-max) rose gradually from winter (2-8 nM h(-1)) to late spring (100-400 nM h-1), which was mainly due to an increase in specific phytoplankton (from 0.02 to 3.0 nmol mu gC(-1) h(-1)) and bacterial APA (from 0.04 to 4.0 nmol mu gC(-1) h(-1)), a strategy to compensate for the lack of phosphate. At each season, both communities had equal competitive abilities to exploit DOP but, taking into account biomass, the phytoplankton community activity always dominated (57-63% of total APA) that of bacterial community (9-11%). The dissolved APA represented a significant contribution. In situ regulation of phytoplanktonic APA by phosphate (induction or inversely repression of enzyme synthesis) was confirmed by simultaneously conducted phosphate-enrichment bioassays. Such changes recorded at a time scale of a few days could partly explain the seasonal response of phytoplankton communities to phosphate depletion.

A new Fine Scale Sampler (FSS) was designed and used in the Loire River plume (Bay of Biscay, France) in order to study phytoplankton distribution in highly stratified water columns. The FSS consists of a linear array of 15 sampling bottles, set horizontally at 20 cm intervals. Data acquired from the CTD and fluorescence probe of the FSS enable it to be precisely positioned at the depth of the highest density gradient or at the chlorophyll a maximum. The FSS made it possible to investigate fine scale vertical distribution of phytoplankton and relationships with water density, nutrient concentrations or light availability, for spring and summer conditions. Under spring conditions, the entire upper layer exhibited a dominant diatom population (Chaetoceros sociale, Thalassiosira rotula) above 15 meters with evident uptake in dissolved inorganic nutrients and the maximum concentration of dinoflagellates (mainly small forms of Gymnodinium spp. and Gyrodinium spp.) was located 16 meters deep, where light energy began to be a limiting factor for phytoplankton growth. During summer conditions, the maximum phytoplankton biomass (10 µg l -1 of chlorophyll a) was only concentrated in the 2 m thick pycnocline layer, located at a depth of 12 m. The diatom Chaetoceros sociale was dominant in the phytoplankton peak. Dinoflagellates (Dinophysis acuminata and Gymnodinium spp.) were also concentrated around the density gradient. Low inorganic nitrogen concentration (< 0.2 µmol.l -1 ) was responsible for the limitation of phytoplankton production in the surface layer above the pycnocline, and the phytoplankton growth was irradiance-limited underneath the pycnocline.